Employer wants me to return to my old job because everyone is quitting. They offered no compensation. (CA). Could use some advice.

I got a promotion at work 2 years ago. This promotion is a totally new job and lets me work from home. Company calls me on Friday and says they want me to do my old job again . They offered no compensation, and no end date. Just two days before this request I signed a job description and a performance evaluation . I asked them for a raise and because the whole company is in union vote, they wouldn’t even have the conversation with me. Now they want to change my job right before a union vote. Is there a way for me to say no? Can they fire me for that? I need some advice. They tell me it’s temporary, they tell me I can do the additional work from home. I just don’t trust them at all. Help? I’m in CA.